Sample inspection system

ABSTRACT

A curved mirrored surface is used to collect radiation scattered by a sample surface and originating from a normal illumination beam and an oblique illumination beam. The collected radiation is focused to a detector. Scattered radiation originating from the normal and oblique illumination beams may be distinguished by employing radiation at two different wavelengths, by intentionally introducing an offset between the spots illuminated by the two beams or by switching the normal and oblique illumination beams on and off alternately. Beam position error caused by change in sample height may be corrected by detecting specular reflection of an oblique illumination beam and changing the direction of illumination in response thereto. Butterfly-shaped spatial filters may be used in conjunction with curved mirror radiation collectors to restrict detection to certain azimuthal angles.

BACKGROUND OF THE INVENTION

[0001] This invention relates in general to sample inspection systemsand, in particular, to an improved inspection system with goodsensitivity for particles as well as crystal-originated-particles(COPs). COPs are surface breaking defects in semiconductor wafers whichhave been classified as particles due to inability of conventionalinspection systems to distinguish them from real particles.

[0002] Systems for inspecting unpatterned wafers or bare wafers havebeen proposed. See for example, PCT Patent Application No.PCT/US96/15354, filed on Sep. 25, 1996, entitled “Improved System forSurface Inspection.” Systems such as those described in theabove-referenced application are useful for many applications, includingthe inspection of bare or unpatterned semiconductor wafers.Nevertheless, it may be desirable to provide improved sample inspectiontools which may be used for inspecting not only bare or unpatternedwafers but also rough films. Another issue which has great significancein wafer inspection is that of COPs. These are surface-breaking defectsin the wafer. According to some opinions in the wafer inspectioncommunity, such defects can cause potential detriments to theperformance of semiconductor chips made from wafers with such defects.It is, therefore, desirable to provide an improved sample inspectionsystem capable of detecting COPs and distinguishing COPs from particles.

SUMMARY OF THE INVENTION

[0003] This invention is based on the observation that anomaly detectionemploying an oblique illumination beam is much more sensitive toparticles than to COPs, whereas in anomaly detection employing anillumination beam normal to the surface, the difference in sensitivityto surface particles and COPs is not as pronounced. Anomaly detectionemploying both an oblique illumination beam and a normal illuminationbeam can then be used to distinguish between particles and COPs.

[0004] One aspect of the invention is directed towards an optical systemfor detecting anomalies of a sample, comprising first means fordirecting a first beam of radiation along a first path onto a surface ofthe sample; second means for directing a second beam of radiation alonga second path onto a surface of the sample and a first detector. Thesystem further comprises means including a mirrored surface forreceiving scattered radiation from the sample surface and originatingfrom the first and second beams and for focusing the scattered radiationto said first detector.

[0005] Another aspect of the invention is directed towards an opticalsystem for detecting anomalies of a sample, comprising first means fordirecting a first beam of radiation along a first path onto a surface ofa sample; second means for directing a second beam of radiation along asecond path onto a surface of the sample, said first and second beamsproducing respectively a first and a second illuminated spot on thesample surface, said first and second illuminated spots separated by anoffset. The system further comprises a detector and means for receivingscattered radiation from the first and second illuminated spots and forfocusing the scattered radiation to said detector.

[0006] One more aspect of the invention is directed towards an opticalsystem for detecting anomalies of a sample, comprising a sourcesupplying a beam of radiation at a first and a second wavelength; andmeans for converting the radiation beam supplied by the source into afirst beam at a first wavelength along a first path and a second beam ata second wavelength along a second path onto a surface of a sample. Thesystem further comprises a first detector detecting radiation at thefirst wavelength and a second detector detecting radiation at the secondwavelength; and means for receiving scattered radiation from the samplesurface and originating from the first and second beams and for focusingthe scattered radiation to said detectors.

[0007] Yet another aspect of the invention is directed towards anoptical system for detecting anomalies of a sample, comprising a sourcesupplying a radiation beam; a switch that causes the radiation beam fromthe source to be transmitted towards the sample surface alternatelyalong a first path and a second path; a detector and means for receivingscattered radiation from the sample surface and originating from thebeam along the first and second paths and for focusing the scatteredradiation to said detector.

[0008] Another aspect of the invention is directed towards an opticalsystem for detecting anomalies of a sample, comprising means fordirecting at least one beam of radiation along a path onto a spot on asurface of the sample; a first detector and means for receivingscattered radiation from the sample surface and originating from the atleast one beam and for focusing the scattered radiation to said firstdetector for sensing anomalies. The system further comprises a second,position sensitive, detector detecting a specular reflection of said atleast one beam in order to detect any change in height of the surface ata spot; and means for altering the path of the at least one beam inresponse to the detected change in height of the surface of the spot toreduce position error of the spot caused by change in height of thesurface of the spot.

[0009] Still another aspect of the invention is directed towards anoptical system for detecting anomalies of a sample, comprising means fordirecting at least one beam of radiation along a path onto a spot on asurface of the sample; a first detector and means for collectingscattered radiation from the sample surface and originating from the atleast one beam and for conveying the scattered radiation to said firstdetector for sensing anomalies. The system further comprises a spatialfilter between the first detector and the collecting and conveying meansblocking scattered radiation towards the detector except for at leastone area having a wedge shape.

[0010] One more aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising directing a firstbeam of radiation along a first path onto a surface of the sample;directing a second beam of radiation along a second path onto a sampleof the surface; employing a mirrored surface for receiving scatteredradiation from the sample surface and originating from the first andsecond beams and focusing the scattered radiation to a first detector.

[0011] Yet another aspect of the invention is directed towards anoptical method for detecting anomalies of a sample, comprising directinga first beam of radiation along a first path onto a surface of thesample; directing a second beam of radiation along a second path onto asurface of the sample, said first and second beams producingrespectively a first and a second illuminated spot on the samplesurface, said first and second illuminated spots separated by an offset.The method further comprises receiving scattered radiation from thefirst and second illuminated spots and for focusing the scatteredradiation to a detector.

[0012] An additional aspect of the invention is directed towards anoptical method for detecting anomalies of a sample, comprising supplyinga beam of radiation of a first and a second wavelength; converting theradiation beam into a first beam at a first wavelength along a firstpath and a second beam at a second wavelength along a second path, saidtwo beams directed towards a surface of the sample. The method furthercomprises collecting scattered radiation from the sample surface andoriginating from the first and second beams, focusing the collectedscattered radiation to one or more detectors, and detecting radiation atthe first and second wavelengths by means of said detectors.

[0013] Yet another aspect of the invention is directed towards anoptical method for detecting anomalies of a sample, comprising supplyinga radiation beam, switching alternately the radiation beam between afirst and a second path towards a surface of the sample, receivingscattered radiation from the sample surface and originating from thebeam along the first and second paths, and focusing the scatteredradiation to a detector.

[0014] Another aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising directing atleast one beam of radiation along a path onto a spot on the surface ofthe sample; collecting scattered radiation from the sample surface andoriginating from the at least one beam, and focusing the collectedscattered radiation to a first detector for sensing anomalies. Themethod further comprises detecting a specular reflection of said atleast one beam in order to detect any change in height of the surface atthe spot and altering the path of the at least one beam in response tothe detected change in height of the surface of the spot to reduceposition error of the spot caused by change in height of the surface ofthe spot.

[0015] One more aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising directing atleast one beam of radiation along a path onto a spot on a surface of thesample; collecting scattered radiation from the sample surface andoriginating from the at least one beam, conveying the scatteredradiation to a first detector for sensing anomalies, and blockingscattered radiation towards the detector except for at least one areahaving a wedge shape.

[0016] Still another aspect of the invention is directed towards anoptical system for detecting anomalies of a sample, comprising means fordirecting a beam of radiation along a path at an oblique angle to asurface of the sample; a detector and means including a curved mirroredsurface for collecting scattered radiation from the sample surface andoriginating from the beam and for focusing the scattered radiation tosaid detector.

[0017] One more aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising directing a beamof radiation along a path at an oblique angle to a surface of thesample; providing a curved mirrored surface to collect scatteredradiation from the sample surface and originating from the beam, andfocusing the scattered radiation from the mirrored surface to a detectorto detect anomalies of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIGS. 1A, 1B and 1C are schematic views of normal or obliqueillumination beams illuminating a surface with a particle thereon usefulfor illustrating the invention.

[0019]FIG. 2A is a schematic view of a sample inspection systememploying an ellipsoidal mirror for illustrating one embodiment of theinvention.

[0020]FIG. 2B is a schematic view of a sample inspection systememploying a paraboloidal mirror to illustrate another embodiment of theinvention.

[0021]FIG. 3 is an exploded simplified view of a portion of the systemof FIG. 2A or FIG. 2B to illustrate another aspect of the invention.

[0022]FIG. 4 is a schematic view of a sample inspection system employingtwo different wavelengths for illumination to illustrate yet anotherembodiment of the invention.

[0023]FIGS. 5A and 5B are schematic views of sample inspection systemsillustrating two different embodiments employing switches for switchinga radiation beam between a normal illumination path and an obliqueillumination path to illustrate yet another aspect of the invention.

[0024]FIG. 6 is a schematic view of a beam illuminating a semiconductorwafer surface to illustrate the effect of a change in height of a waferon the position of the spot illuminated by beam.

[0025]FIG. 7 is a schematic view of a portion of a sample inspectionsystem inspecting a semiconductor wafer, employing three lenses, wherethe direction of the illumination beam is altered to reduce the error inthe position of the illuminated spot caused by the change in height ofthe wafer.

[0026]FIG. 8 is a schematic view of a portion of a sample inspectionsystem employing only one lens to compensate for a change in height ofthe wafer.

[0027] FIGS. 9A-9F are schematic views of six different spatial filtersuseful for detecting anomalies of samples.

[0028]FIG. 10A is a simplified partially schematic and partiallycross-sectional view of a programmable spatial filter employing a layerof liquid crystal material sandwiched between an electrode and an arrayof electrodes in the shape of sectors of a circle and means for applyinga potential difference across at least one sector in the array and theother electrode, so that the portion of the liquid crystal layeradjacent to the at least one sector is controlled to be radiationtransparent or scattering.

[0029]FIG. 10B is a top view of the filter of FIG. 10A.

[0030] For simplicity in description, identical components are labelledby the same numerals in this application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031]FIG. 1A is a schematic view of a surface 20 of a sample to beinspected and an illumination beam 22 directed in a direction normal tosurface 20 to illuminate the surface and a particle 24 on the surface.Thus, the illumination beam 22 illuminates an area or spot 26 of surface20 and a detection system (not shown) detects light scattered byparticle 24 and by portion or spot 26 of the surface 20. The ratio ofthe photon flux received by the detector from particle 24 to that fromspot 26 indicates the sensitivity of the system to particle detection.

[0032] If an illumination beam 28 directed at an oblique angle tosurface 20 is used to illuminate spot 26′ and particle 24 instead, asshown in FIG. 1B, from a comparison between FIGS. 1A and 1B, it will beevident that the ratio of the photon flux from the particle 24 to thatfrom the illuminated spot will be greater in the case of the obliqueillumination in FIG. 1B compared to that in FIG. 1A. Therefore, for thesame throughput (spots 26, 26′ having the same area), the sensitivity ofthe oblique incidence beam in detecting small particles is superior andis the method of choice in the detection of small particles.

[0033]FIG. 1C illustrates an oblique beam 28′ illuminating a surface 30having a pit 32 and particle 24′ thereon. As can be seen from FIG. 1C,even though the pit 32 is of comparable size to particle 24, it willscatter a much smaller amount of photon flux compared to particle 24from oblique beam 28′. On the other hand, if the pit 32 and particle 24are illuminated by a beam such as 22 directed in a direction normal tosurface 30, pit 32 and particle 24 would cause comparable amount ofphoton flux scattering. Almost regardless of the exact shape ororientation of COPs and particles, anomaly detection employing obliqueillumination is much more sensitive to particles than COPS. In the caseof anomaly detection with normal illumination, however, thedifferentiation between particles and COPs is less pronounced.Therefore, by means of a simultaneous, or sequential, comparison offeature signatures due to normal and oblique illumination will revealwhether the feature is a particle or a COP.

[0034] Azimuthal collection angle is defined as the angle made by thecollection direction to the direction of oblique illumination whenviewed from the top. By employing oblique illumination, together with ajudicious choice of the azimuthal collection angle, rough films can beinspected with good sensitivity, such as when a spatial filter shown inany of FIGS. 9A-9F, 10A and 10B is used in any one of the embodiments asshown in FIGS. 2A, 2B, 3, 4, 5A and 5B, as explained below. By retainingthe normal illumination beam for anomaly detection, all of theadvantageous attributes of the system described in PCT PatentApplication No. PCT/US96/15354 noted above, are retained, including itsuniform scratch sensitivity and the possibility of adding a bright-fieldchannel as described in PCT Patent Application. No. PCT/US97/04134,filed Mar. 5, 1997, entitled “Single Laser Bright Field and Dark FieldSystem for Detecting Anomalies of a Sample.”

[0035] Scanning a sample surface with oblique and normal illuminationbeams can be implemented in a number of ways. FIG. 2A is a schematicview of a sample inspection system to illustrate a general set up forimplementing anomaly detection using both normal and obliqueillumination beams. A radiation source that provides radiation at one ormore wavelengths in a wide electromagnetic spectrum (including but notlimited to ultraviolet, visible, infrared) may be used, such as a laser52 providing a laser beam 54. A lens 56 focuses the beam 54 through aspatial filter 58 and lens 60 collimates the beam and conveys it to apolarizing beamsplitter 62. Beamsplitter 62 passes a first polarizedcomponent to the normal illumination channel and a second polarizedcomponent to the oblique illumination channel, where the first andsecond components are orthogonal. In the normal illumination channel 70,the first polarized component is focused by optics 72 and reflected bymirror 74 towards a sample surface 76 a of a semiconductor wafer 76. Theradiation scattered by surface 76 a is collected and focused by anellipsoidal mirror 78 to a photomultiplier tube 80.

[0036] In the oblique illumination channel 90, the second polarizedcomponent is reflected by beamsplitter 62 to a mirror 82 which reflectssuch beam through a half-wave plate 84 and focused by optics 86 tosurface 76 a. Radiation originating from the oblique illumination beamin the oblique channel 90 and scattered by surface 76 a is collected byan ellipsoidal mirror and focused to photomultiplier tube 80.Photomultiplier tube 80 has a pinhole entrance 80 a. The pinhole 80 aand the illuminated spot (from the normal and oblique illuminationchannels on surface 76 a) are preferably at the foci of the ellipsoidalmirror 78.

[0037] Wafer 76 is rotated by a motor 92 which is also moved linearly bytransducer 94, and both movements are controlled by a controller 96, sothat the normal and oblique illumination beams in channels 70 and 90scan surface 76 a along a spiral scan to cover the entire surface.

[0038] Instead of using an ellipsoidal mirror to collect the lightscattered by surface 76 a, it is also possible to use other curvedmirrors, such as a paraboloidal mirror 78′ as shown in system 100 ofFIG. 2B. The paraboloidal mirror 78′ collimates the scattered radiationfrom surface 76 a into a collimated beam 102 and the collimated beam 102is then focused by an objective 104 and through an analyzer 98 to thephotomultiplier tube 80. Aside from such difference, the sampleinspection system 100 is exactly the same as system 50 of FIG. 2A.Curved mirrored surfaces having shapes other than ellipsoidal orparaboloidal shapes may also be used; preferably, each of such curvedmirrored surfaces has an axis of symmetry substantially coaxial with thepath of the normal illumination path, and defines an input aperture forreceiving scattered radiation. All such variations are within the scopeof the invention. For simplicity, the motor, transducer and control formoving the semiconductor wafer has been omitted from FIG. 2B and fromFIGS. 4, 5A, 5B described below.

[0039] The general arrangements shown in FIGS. 2A and 2B can beimplemented in different embodiments. Thus, in one arrangement referredto below as the “GO and RETURN” option, a half-wave plate (not shown) isadded between laser 52 and lens 56 in FIGS. 2A and 2B so that thepolarization of the light reaching the beamsplitter 62 can be switchedbetween P and S. Thus, during the Go cycle, the beamsplitter 62 passesradiation only into the normal channel 70 and no radiation will bedirected towards the oblique channel 90. Conversely, during the RETURNcycle, beamsplitter 62 passes radiation only into the oblique channel 90and no radiation will be directed through the normal channel 70. Duringthe GO cycle, only the normal illumination beam 70 is in operation, sothat the light collected by detector 80 is recorded as that from normalillumination. This is performed for the entire surface 76 a where motor92, transducer 94 and control 96 are operated so that the normalillumination beam 70 scans the entire surface 76 a along a spiral scanpath.

[0040] After the surface 76 a has been scanned using normalillumination, the half-wave plate between laser 52 and lens 56 causesradiation from laser 52 to be directed only along the oblique channel 90and the scanning sequence by means of motor 92, transducer 94 andcontrol 96 is reversed and data at detector 80 is recorded in a RETURNcycle. As long as the forward scan in the GO cycle and the reverse scanin the RETURN cycle are exactly registered, the data set collectedduring the GO cycle and that collected during the return cycle may becompared to provide information concerning the nature of the defectsdetected. Instead of using a half-wave plate and a polarizingbeamsplitter as in FIG. 2A, the above-described operation may also beperformed by replacing such components with a removable mirror placed inthe position of beamsplitter 62. If the mirror is not present, theradiation beam from laser 52 is directed along the normal channel 70.When the mirror is present, the beam is then directed along the obliquechannel 90. Such mirror should be accurately positioned to ensure exactregistration of the two scans during the Go and RETURN cycles. Whilesimple, the above-described GO and RETURN option requires extra timeexpended in the RETURN cycle.

[0041] The normal illumination beam 70 illuminates a spot on surface 76a. The oblique illumination beam 90 also illuminates a spot on thesurface 76 a. In order for comparison of data collected during the twocycles to be meaningful, the two illuminated spots should have the sameshape. Thus, if beam 90 has a circular cross-section, it wouldilluminate an elliptical spot on the surface. In one embodiment,focusing optics 72 comprises a cylindrical lens so that beam 70 has anelliptical cross-section and illuminates also an elliptical spot onsurface 76 a.

[0042] To avoid having to scan surface 76 a twice, it is possible tointentionally introduce a small offset between the illuminated spot 70 afrom normal illumination beam 70 (referred to herein as “normalillumination spot” for simplicity) and the illuminated spot 90 a fromoblique illumination beam 90 (referred to herein as “obliqueillumination spot” for simplicity) as illustrated in FIG. 3. FIG. 3 isan enlarged view of surface 76 a and the normal and oblique illuminationbeams 70, 90 to illustrate an offset 120 between the normal and obliqueillumination spots 70 a, 90 a. In reference to FIGS. 2A, 2B, radiationscattered from the two spots 70 a, 90 a would be detected at differenttimes and would be distinguished.

[0043] The method illustrated in FIG. 3 causes a reduction in systemresolution and increased background scattering due to the presence ofboth spots. In other words, in order that radiation scattered from bothspots separated by an offset would be focused through pinhole 80 a, thepinhole should be somewhat enlarged in the direction of the offset. As aconsequence, detector 80 will sense an increased background scatteringdue to the enlargement of the pinhole 80 a. Since the background is dueto both beams whereas the particle scattered radiation is due to one orthe other spot, the signal-to-noise ratio is decreased. Preferably, theoffset is not greater than three times the spatial extent, or less thanthe spatial extent, of the point spread function of either the normal oroblique illumination beam. The method illustrated in FIG. 3, however, isadvantageous since throughput is not adversely affected compared to thatdescribed in PCT Application No. PCT/US96/15354 and the Censor ANSseries of inspection systems from KLA-Tencor Corporation of San Jose,Calif., the assignee of this application.

[0044]FIG. 4 is a schematic view of a sample inspection system employinga normal illumination beam comprising radiation at a first wavelength λ₁and an oblique illumination beam of radiation of wavelength λ₂ toillustrate another embodiment of the invention. The laser 52 of FIGS.2A, 2B may supply radiation at only one wavelength, such as 488 nm ofargon. Laser 52′ of FIG. 4 supplies radiation at at least two differentwavelengths in beam 54′, such as at 488 and 514 nm, instead of radiationof only one wavelength, Such beam is split by a dichroic beamsplitter162 into a first beam at a first wavelength λ₁ (488 nm) and a secondbeam of wavelength λ₂ (514 nm), by passing radiation at wavelength λ₁and reflecting radiation at wavelength λ₂, for example. After beingfocused by optics 72, beam 70′ at wavelength λ₁ is reflected by mirror74 towards surface 76 a as the normal illumination beam. The reflectedradiation of wavelength λ₂ at beamsplitter 162 is further reflected bymirror 82 and focused by optics 86 as the oblique illumination beam 90′to illuminate the surface. The optics in both the normal and obliqueillumination paths are such that the normal and oblique illuminatedspots substantially overlap with no offset there between. The radiationscattered by surface 76 a retains the wavelength characteristics of thebeams from which the radiation originate, so that the radiationscattered by the surface originating from normal illumination beam 70′can be separated from radiation scattered by the surface originatingfrom oblique illumination beam 90′. Radiation scattered by surface 76 ais again collected and focused by an ellipsoidal mirror 78 through apinhole 164 a of a spatial filter 164 to a dichroic beamsplitter 166. Inthe embodiment of FIG. 4, beamsplitter 166 passes the scatteredradiation at wavelength λ₁ to detector 80(1) through a lens 168.Dichroic beamsplitter 166 reflects scattered radiation at wavelength 2through a lens 170 to photomultiplier tube 80(2). Again, the mechanismfor causing the wafer to rotate along a spiral path has been omittedfrom FIG. 4 for simplicity.

[0045] Instead of using a laser that provides radiation at a singlewavelength, the laser source 52′ should provide radiation at twodistinct wavelengths. A commercially available multi-line laser sourcethat may be used is the 2214-65-ML manufactured by Uniphase, San Jose,Calif. The amplitude stability of this laser at any given wavelength isaround 3.5%. If such a laser is used, the scheme in FIG. 4 will beuseful for applications such as bare silicon inspection but may havediminished particle detection sensitivity when used to scan rough films.

[0046] Yet another option for implementing the arrangements generallyshown in FIGS. 2A and 2B is illustrated in FIGS. 5A and 5B. In suchoption, a radiation beam is switched between the normal and obliqueillumination channels at a higher frequency than the data collectionrate so that the data collected due to scattering from the normalillumination beam may be distinguished from data collected fromscattering due to the oblique illumination channel. Thus as shown inFIG. 5A, an electro-optic modulator (e.g. a Pockels cell) 182 is placedbetween laser 52 and beamsplitter 62 to modulate the radiation beam 54at the half-wave voltage. This results in the beam being eithertransmitted or reflected by the polarizing beamsplitter 62 at the drivefrequency of modulator 182 as controlled by a control 184.

[0047] The electro-optic modulator may be replaced by a Bragg modulator192 as shown in FIG. 5B, which may be turned on and off at a highfrequency as controlled modulator 192 is powered by block 193 atfrequency ω_(b). This block is turned on and off at a frequency ω_(m).In the off condition, a zero order beam 194 a passes through the Braggmodulator 192, and becomes the normal illumination beam reflected tosurface 76 a by mirror 74. In the on condition, cell 192 generates adeflected first order beam 194 b, which is reflected by mirrors 196, 82to surface 76 a. However, even though most of the energy from cell 192is directed to the oblique first order beam, a weak zero order normalillumination beam is still maintained, so that the arrangement in FIG.5B is not as good as that in FIG. 5A.

[0048] Preferably, the electro-optic modulator of FIG. 5A and the Braggmodulator of FIG. 5B are operated at a frequency higher than the datarate, and preferably, at a frequency at least about 3 or 5 times thedata rate of tube 80. As in FIG. 4, the optics in both the normal andoblique illumination paths of FIGS. 5A, 5B are such that the normal andoblique illuminated spots substantially overlap with no offset therebetween. The arrangements in FIGS. 2A, 2B, 4, 5A, 5B are advantageous inthat the same radiation collector 78 and detector 80 are used fordetecting scattered light originating from the normal illumination beamas well as from the oblique illumination beam. Furthermore, by employinga curved surface that collects radiation that is scattered within therange of at least 25 to 70° from a normal direction to surface 76 a andfocusing the collected radiation to the detector, the arrangements ofFIGS. 2A, 2B, 4, 5A, 5B maximize the sensitivity of detection.

[0049] In contrast to arrangements where multiple detectors are placedat different azimuthal collection angles relative to the obliqueillumination beam, the arrangements of FIGS. 2A, 2B has superiorsensitivity and is simpler in arrangement and operation, since there isno need to synchronize or correlate the different detection channelsthat would be required in a multiple detector arrangement. Theellipsoidal mirror 78 collects radiation scattered within the range ofat least 25 to 70° from the normal direction to the surface whichaccounts for most of the radiation that is scattered by surface 76 afrom an oblique illumination beam, and that contains information usefulfor particle and COPs detection.

[0050] The three dimensional intensity distribution of scatteredradiation from small particles on the surface when the surface isilluminated by a P-polarized illumination beam at or near a grazingangle to the surface has the shape of a toroid. In the case of largeparticles, higher scattered intensity is detected in the forwarddirection compared to other directions. For this reason, the curvedmirror collectors of FIGS. 2A, 2B, 4, 5A, 5B are particularlyadvantageous for collecting the scattered radiation from small and largeparticles and directing the scattered radiation towards a detector. Inthe case of normal illumination, however, the intensity distribution ofradiation scattered from small particles on surfaces is in the shape ofa sphere. The collectors in FIGS. 2A, 2B, 4, 5A, 5B are alsoadvantageous for collecting such scattered radiation. Preferably, theillumination angle of beam 90 is within the range of 45 to 85° from anormal direction to the sample surface, and preferably at 70 or 75°,which is close to the principal angle of silicon at 488 and 514 nm, andwould allow the beam passage to be unhindered by the walls of thecollector. By operating at this shallow angle, the particle photon fluxis enhanced as illustrated in FIGS. 1A and 1B and the discriminationagainst the pits is substantial.

[0051] Beam Position Correction

[0052] A prerequisite for the comparison of signals generated by twodetection channels for a given defect is the ability to place the twospots on the same location. In general, semiconductor wafers or othersample surfaces are not completely flat, nor do they have the samethickness. Such imperfections are of little concern for anomalydetection employing a normal incidence beam, as long as the wafersurface remains within the depth of focus. In the case of the obliqueillumination beam, however, wafer-height variation will cause the beamposition and hence the position of the illuminated spot to be incorrect.In FIG. 6, θ is the oblique incidence angle between the beam and anormal direction N to the wafer surface. Thus, as shown in FIG. 6, ifthe height of the wafer surface moves from the dotted line position 76a′ to the solid line position 76 a which is higher than the dotted lineposition by the height h, then the position of the illuminated spot onthe wafer surface will be off by an error of w given by h.tan θ. Onepossible solution is to detect the change in height of the wafer at theilluminated spot and move the wafer in order to maintain the wafer at aconstant height at the illuminated spot, as described in U.S. Pat. No.5,530,550. In the embodiment described above, the wafer is rotated andtranslated to move along a spiral scan path so that it may be difficultto also correct the wafer height by moving the wafer while it is beingrotated along such path. Another alternative is to move the light sourceand the, detector when the height of the wafer changes so as to maintaina constant height between the light source and the detector on the onehand and the wafer surface at the illuminated spot on the other. This isobviously cumbersome and may be impractical. Another aspect of theinvention is based on the observation that, by changing the direction ofthe illumination beam in response to a detected change in wafer height,it is possible to compensate for the change in wafer height to reducebeam position error caused thereby.

[0053] One scheme for implementing the above aspect is illustrated inFIG. 7. As shown in system 200 of FIG. 7, an illumination beam isreflected by a mirror 202 and focused through three lenses L₁, L₂, L₃ tothe wafer surface 204 a. The positions of the lenses are set in order tofocus an oblique illumination beam 70″ to wafer surface 204 a in dottedline in FIG. 7. Then a quad cell (or other type of position sensitivedetector) 206 is positioned so that the specular reflection 70 a″ of thebeam 70″ from surface 204 reaches the cell at the null or zero position206 a of the cell. As the wafer surface moves from position 204 a to 204b shown in solid line in FIG. 7, such change in height of the wafercauses the specular reflection to move to position 70 b″, so that itreaches the cell 206 at a position on the cell offset from the nullposition 206 a. Detector 206 may be constructed in the same manner asthat described in U.S. Pat. No. 5,530,550. A position error signaloutput from detector 206 indicating the deviation from the null positionin two orthogonal directions is sent by cell. 206 to a control 208 whichgenerates an error signal to a transducer 210 for rotating the mirror202 so that the specular reflection 70 b″ also reaches the cell at thenull position 206 a. In other words, the direction of the illuminationbeam is altered until the specular reflection reaches the cell at nullposition, at which point control 208 applies no error signal to thetransducer 210.

[0054] Instead of using three lenses, it is possible to employ a singlelens as shown in FIG. 8, except that the correct placement of theilluminated spot on the wafer corresponds not to a null in the positionsensing signal from the position sensitive detector, but corresponds toan output of the detector reduced by ½. This approach is shown in FIG.8. Thus, controller 252 divides by 2 the amplitude of the positionsensing signal at the output of quad cell detector 254 to derive aquotient signal and applies the quotient signal to transducer 210. Thetransducer 210 rotates the mirror by an amount proportional to theamplitude of the quotient signal. The new position of the specularreflection corresponds to the correct location of the spot. The newerror signal is now the new reference.

[0055] The above described feature of reducing beam position error ofthe oblique illumination beam in reference to FIGS. 7 and 8 may be usedin conjunction with any one of the inspection systems of FIGS. 2A, 2B,3, 4, 5A and 5B, although only the quad cell (206 or 254) is shown inthese figures.

[0056] Spatial Filter

[0057] In reference to the embodiments of FIGS. 2A, 2B, 4, 5A and 5B, itis noted that the radiation collection and detection schemes in suchembodiments retain the information concerning the direction ofscattering of the radiation from surface 76 a relative to the obliqueillumination channel 90 or 90′. This can be exploited for someapplications such as rough surface inspection. This can be done byemploying a spatial filter which blocks the scattered radiationcollected by the curved mirrored surface towards the detector except forat least one area have a wedge shape. With respect to the normalillumination channel, there is no directional information since both theillumination and scattering are symmetrical about a normal to thesurface. In other words, if the normal illumination channel is omittedin the embodiments of FIGS. 2A, 2B, 4, 5A and 5B, the curved mirroredcollector 78 or 78′ advantageously collects most of the radiationscattered within the toroidal intensity distribution caused by particlescattering to provide an inspection tool of high particle sensitivity.At the same time, the use of a curved mirrored collector retains thedirectional scattering information, where such information can beretrieved by employing a spatial filter as described below.

[0058] FIGS. 9A-9F illustrate six different embodiments of such spatialfilters in the shape of butterflies each with two wings. The dark orshaded areas (wings) in these figures represent areas that are opaque toor scatters radiation, and the white or unshaded areas represent areasthat transmit such radiation. The size(s) of the radiation transmissive(white or unshaded) area(s) are determined in each of the filters inFIGS. 9A-9F by the wedge angle α. Thus, in FIG. 9A, the wedge angle is10°, whereas in FIG. 9B, it is 20°.

[0059] Thus, if the filter in FIG. 9B is placed at position 300 of FIGS.2A, 2B, 4, 5A or 5B where the 20° wedge-shaped area of radiationcollection is centered at approximately 90° and 270° azimuthalcollection angles relative to the oblique illumination direction, thishas the effect of generating a combined output from two detectors, eachwith a collection angle of 20°, one detector placed to collect radiationbetween 80 to 100° azimuthal angles as in U.S. Pat. No. 4,898,471, andthe other detector to collect radiation between 260 and 280° azimuthalangles. The detection scheme of U.S. Pat. No. 4,898,471 can be simulatedby blocking out also the wedge area between 260 and 280 azimuthalangles. The arrangement of this application has the advantage over U.S.Pat. No. 4,898,471 of higher sensitivity since more of the scatteredradiation is collected than in such patent, by means of the curvedmirror collector 78, 78′. Furthermore, the azimuthal collection anglecan be dynamically changed by programming the filter at position 300 inFIGS. 2A, 2B, 4, 5A, 5B without having to move any detectors, asdescribed below.

[0060] It is possible to enlarge or reduce the solid angle of collectionof the detector by changing α. It is also possible to alter theazimuthal angles of the wedge areas. These can be accomplished by havingready at hand a number of different filters with different wedge anglessuch as those shown in FIGS. 9A-9F, as well as filters with other wedgeshaped radiation transmissive areas, and picking the desired filter andthe desired position of the filter for use at position 300 in FIGS. 2A,2B, 4, 5A, 5B. The spatial filters in FIGS. 9A-9E are all in the shapeof butterflies with two wings, where the wings are opaque to or scattersradiation and the spaces between the wings transmit radiation betweenthe mirrored surfaces and detector 80. In some applications, however, itmay be desirable to employ a spatial filter of the shape shown in FIG.9F having a single radiation transmissive wedge-shaped area. Obviously,spatial filters having any number of wedge-shaped areas that areradiation transmissive dispersed around a center at various differentangles may also be used and are within the scope of the invention.

[0061] Instead of storing a number of filters having different wedgeangles, different numbers of wedges and distributed in variousconfigurations, it is possible to employ a programmable spatial filterwhere the opaque or scattering and transparent or transmissive areas maybe altered. For example, the spatial filter may be constructed usingcorrugated material where the wedge angle α can be reduced by flatteningthe corrugated material. Or, two or more filters such as those in FIGS.9A-9F may be superimposed upon one another to alter the opaque orscattering and transparent or transmissive areas.

[0062] Alternatively, a liquid crystal spatial filter may beadvantageously used, one embodiment of which is shown in FIGS. 10A and10B. A liquid crystal material can be made radiation transmissive orscattering by changing an electrical potential applied across the layer.The liquid crystal layer may be placed between a circular electrode 352and an electrode array 354 in the shape of n sectors of a circlearranged around a center 356, where n is a positive integer. The sectorsare shown in FIG. 10B which is a top view of one embodiment of filter350 in FIG. 10A Adjacent electrode sectors 354(i) and 354(i+1), Iranging from 1 to n−1, are electrically insulated from each other.

[0063] Therefore, by applying appropriate electrical potentials acrossone or more of the sector electrodes 354(i), where (i) ranges from 1 ton, on one side, and electrode 352 on the other side, by means of voltagecontrol 360, it is possible to programmably change the wedge angle α byincrements equal to the wedge angle β of each of the sector electrodes354(1) through 354(n). By applying the potentials across electrode 352and the appropriate sector electrodes, it is also possible to achievefilters having different numbers of radiation transmissive wedge-shapedareas disposed in different configurations around center 356, again withthe constraint of the value of β. To simplify the drawings, theelectrical connection between the voltage control 360 and only one ofthe sector electrodes is shown in FIGS. 10A and 10B. Instead of being inthe shape of sectors of a circle, electrodes 354 can also be in theshape of triangles. Where electrodes 354 are shaped as isoscelestriangles, the array of electrodes 354 arranged around center 356 hasthe shape of a polygon. Still other shapes for the array 354 arepossible.

[0064] If the wedge angle β is chosen to be too small, this means thatan inordinate amount of space must be devoted to the separation betweenadjacent sector electrodes to avoid electrical shorting. Too large avalue for β means that the wedge angle α can only be changed by largeincrements. Preferably β is at least about 5°.

[0065] For the normal illumination beam, the polarization state of thebeam does not, to first order, affect detection. For the obliqueillumination beam, the polarization state of the beam can significantlyaffect detection sensitivity. Thus, for rough film inspection, it may bedesirable to employ S polarized radiation, whereas for smooth surfaceinspection, S or P polarized radiation may be preferable. After thescattered radiation from the sample surface originating from each of thetwo channels have been detected, the results may be compared to yieldinformation for distinguishing between particles and COPs. For example,the intensity of the scattered radiation originating from the obliquechannel (e.g., in ppm) may be plotted against that originating from thenormal channel, and the plot is analyzed. Or a ratio between the twointensities is obtained for each of one or more locations on the samplesurface. Such operations may be performed by a processor 400 in FIGS.2A, 2B, 4, 5A, 5B.

[0066] While the invention has been described by reference to a normaland an oblique illumination beam, it will be understood that the normalillumination beam may be replaced by one that is not exactly normal tothe surface, while retaining most of the advantages of the inventiondescribed above. Thus, such beam may be at a small angle to the normaldirection, where the small angle is no more than 10° to the normaldirection.

[0067] While the invention has been described above by reference tovarious embodiments, it will be understood that changes andmodifications may, be made without departing from the scope of theinvention, which is to be defined only by the appended claims and theirequivalents. For example, while only two illuminating beams or paths areshown in FIGS. 2A, 2B, 4, 5A, 5B, it will be understood that three ormore illuminating beams or paths may be employed and are within thescope of the invention.

What is claimed is:
 1. An optical system for detecting anomalies of asample, comprising: first means for directing a first beam of radiationalong a first path onto a surface of the sample; second means fordirecting a second beam of radiation along a second path onto a surfaceof the sample; a first detector; and means including a mirrored surfacefor receiving scattered radiation from the sample surface andoriginating from the first and second beams and for focusing thescattered radiation to said first detector.
 2. The system of claim 1,wherein said mirrored surface is a curved surface and has an axis ofsymmetry substantially coaxial with the first path, defining an inputaperture positioned proximate to the sample surface to receive scatteredradiation therethrough from the sample surface.
 3. The system of claim2, said mirrored surface being a paraboloidal mirrored surface, themirrored surface reflecting radiation that passes through the inputaperture, said receiving and focusing means further including means forfocusing radiation reflected by the mirrored surface to the firstdetector.
 4. The system of claim 2, said mirrored surface being anellipsoidal mirrored surface, the mirrored surface reflecting andfocusing radiation that passes through the input aperture.
 5. The systemof claim. 1, said first path being not more than about 10° angle from anormal direction to the sample surface.
 6. The system of claim 5, saidfirst path being substantially normal to the sample surface.
 7. Thesystem of claim 5, said second path being at an angle within a range ofabout 45 to 85 degrees to a normal direction to the sample surface. 8.The system of claim 1, said first and second beams producing a first anda second illuminated spot on the sample surface, said first and secondilluminated spots substantially coinciding.
 9. The system of claim 1,said first and second beams producing a first and a second illuminatedspot on the sample surface, said first and second illuminated spotsseparated by an offset.
 10. The system of claim 9, said first or secondbeam having a point spread function with a spatial extent, wherein saidoffset is not less than said spatial extent but not greater than threetimes the spatial extent.
 11. The system of claim 1, said first andsecond means comprising: a source supplying a radiation beam; and meansfor converting the radiation beam supplied by the source into said firstand second beams.
 12. The system of claim 11, said source supplyingradiation of at least a first and a second wavelength, wherein saidfirst detector detects radiation of the first wavelength, said systemfurther comprising a second detector for detecting radiation of thesecond wavelength.
 13. The system of claim 11, said converting meansincluding a switch that causes the radiation beam from the source to betransmitted alternately along the two paths towards the sample surface.14. The system of claim 13, said system further comprising means foracquiring data from the detector at a data rate, said switch operatingat a frequency of at least about three times that of the data rate. 15.The system of claim 13, said system further comprising means foracquiring data from the detector at a data rate, said switch operatingat a frequency of at least about five times that of the data rate. 16.The system of claim 13, said switch including an electro-optic modulatoror Bragg modulator.
 17. The system of claim 1, the sample having asmooth surface, wherein the second path is at an oblique angle to thesample surface, and the second beam is P or S polarized with respect tothe sample surface.
 18. The system of claim 1, the sample having a roughsurface, wherein the second path is at an oblique angle to the samplesurface, and the second beam is S polarized with respect to the samplesurface.
 19. The system of claim 1, further comprising means forcomparing detected scattered radiation originating from the first beamand that originating from the second beam to distinguish betweenparticles and COPS.
 20. An optical system for detecting anomalies of asample, comprising: first means for directing a first beam of radiationalong a first path onto a surface of the sample; second means fordirecting a second beam of radiation along a second path onto a surfaceof the sample, said first and second beams producing a first and asecond illuminated spot on the sample surface, said first-and secondilluminated spots separated by an offset; a detector; and means forreceiving scattered radiation from the first and second illuminatedspots and for focusing the scattered radiation to said detector.
 21. Thesystem of claim 20, said first or second beam having a point spreadfunction with a spatial extent, wherein said offset is not less thansaid spatial extent but not greater than three times the spatial extent.22. An optical system for detecting anomalies of a sample, comprising: asource supplying a beam of radiation at at least a first and a secondwavelength; and means for converting the radiation beam supplied by thesource into a first beam at a first wavelength along a first path and asecond beam at a second wavelength along a second path onto a surface ofthe sample; a first detector detecting radiation at the first wavelengthand a second detector detecting radiation at the second wavelength; andmeans for receiving scattered radiation from the sample surface andoriginating from the first and second beams and for focusing thescattered radiation to said detectors.
 23. An optical system fordetecting anomalies of a sample, comprising: a source supplying aradiation beam; a switch that causes the radiation beam from the sourceto be transmitted towards the sample surface alternately along a firstand a second path; a detector; means for receiving scattered radiationfrom the sample surface and originating from the beam along the firstand second paths and for focusing the scattered radiation to saiddetector.
 24. The system of claim 23, said system further comprisingmeans for acquiring data from the detector at a data rate, said switchoperating at a frequency of at least about three times that of the datarate.
 25. The system of claim 23, said switch including an electro-opticmodulator or Bragg modulator.
 26. An optical system for detectinganomalies of a sample, comprising: means for directing at least one beamof radiation along a path onto a spot on a surface of the sample; afirst detector; means for receiving scattered radiation from the samplesurface and originating from the at least one beam and for focusing thescattered radiation to said first detector for sensing anomalies; asecond, position sensitive, detector detecting a specular reflection ofsaid at least one beam in order to detect any change in height of thesurface at the spot; and means for altering the path of the at least onebeam in response to the detected change in height of the surface at thespot to reduce position error of the spot caused by change in height ofthe surface at the spot.
 27. The system of claim 26, said directingmeans including a mirror, said altering means including means forrotating the mirror.
 28. The system of claim 27, said directing meansincluding three lenses focusing a beam from the mirror to the samplesurface.
 29. The system of claim 26, said second detector providing aposition signal indicating height of the sample surface at the spot,said altering means including: means for providing a control signalhaving an amplitude equal to about half that of the position signal; anda transducer rotating the mirror by an amount proportional to theamplitude of the control signal.
 30. The system of claim 29, saidproviding means including a controller.
 31. The system of claim 27, saiddirecting means including a lens focusing a beam from the mirror to thesample surface.
 32. An optical system for detecting anomalies of asample, comprising: means for directing at least one beam of radiationalong a path onto a spot on a surface of the sample; a first detector;means for collecting scattered radiation from the sample surface andoriginating from the at least one beam and for conveying the scatteredradiation to said first detector for sensing anomalies; and a spatialfilter between the first detector and the collecting and conveying meansblocking scattered radiation towards the detector except for at leastone area having a wedge shape.
 33. The system of claim 32, said filterhaving a shape of a butterfly with two wings, wherein scatteredradiation towards the detector is blocked by the two wings and passesbetween the wings.
 34. The system of claim 32, wherein said filter isprogrammable to alter the size, position or orientation of the at leastone area.
 35. The system of claim 34, said wedge shape having a wedgeangle, said filter comprising: an array of wedge-shaped electrodesarranged around a center, each electrode in the array overlapping atleast one additional electrode; a layer of liquid crystal materialbetween the array and said at least one additional electrode; and meansfor applying electrical potentials across one or more electrodes in thearray and the at least one additional electrode to control radiationtransmission through sections of the liquid crystal layer to alter thesize, position or orientation of the at least one area.
 36. The systemof claim 35, said wedge-shape electrodes having wedge angle(s) that areat least about 5 degrees.
 37. The system of claim 32, said directingmeans directing said at least one beam of radiation along a path at anoblique angle to the sample surface.
 38. An optical method for detectinganomalies of a sample, comprising: directing a first beam of radiationalong a first path onto a surface of the sample; directing a second beamof radiation along a second path onto a surface of the sample; andemploying a mirrored surface for receiving scattered radiation from thesample surface and originating from the first and second beams andfocusing the scattered radiation to a first detector.
 39. The method ofclaim 38, said first path being not more than about 10° angle from anormal direction to the sample surface.
 40. The method of claim 38, saidfirst path being substantially normal to the sample surface.
 41. Themethod of claim 38, said second path being at an angle within a range ofabout 45 to 85 degrees to a normal direction to the sample surface. 42.The method of claim 38, said first and second beams producing a firstand a second illuminated spot on the sample surface, said first andsecond illuminated spots substantially coinciding.
 43. The method ofclaim 38, said first and second beams producing a first and a secondilluminated spot on the sample surface, said first and secondilluminated spots separated by an offset.
 44. The method of claim 43,said first or second beam having a point spread function with a spatialextent, wherein said offset is not less than said spatial extent but notgreater than three times the spatial extent.
 45. The method of claim 38,said first and second beam directing comprising: providing a sourcesupplying a radiation beam; and converting the radiation beam suppliedby the source into said first and second beams.
 46. The method of claim45, said source supplying radiation of a first and a second wavelength,wherein said first detector detects radiation of the first wavelength,said method further comprising detecting radiation of the secondwavelength by means of a second detector.
 47. The method of claim 45,said converting including switching the radiation beam from the sourcealternately between the two paths towards the sample surface.
 48. Themethod of claim 47, said method further comprising acquiring data fromthe first detector at a data rate, wherein said switching is at afrequency of at least about three times that of the data rate.
 49. Themethod of claim 47, said method further comprising acquiring data fromthe first detector at a data rate, wherein said switching is at afrequency of at least about five times that of the data rate.
 50. Themethod of claim 38, the sample having a smooth surface, wherein thesecond path is at an oblique angle to the sample surface, and thedirecting directs a second beam that is S or P polarized with respect tothe sample surface.
 51. The method of claim 38, the sample having arough surface, wherein the second path is at an oblique angle to thesample surface, and the directing directs a second beam that is Spolarized with respect to the sample surface.
 52. The method of claim38, further comprising scanning sequentially the first and second beamsacross the same portion of the sample surface, wherein the first but notthe second beam is directed to said surface while it is being scanned ina cycle, and the second but not the first beam is directed to saidsurface while it is being scanned in a subsequent cycle.
 53. The methodof claim 38, further comprising comparing detected scattered radiationoriginating from the first beam and that originating from the secondbeam to distinguish between particles and COPs.
 54. An optical methodfor detecting anomalies of a sample, comprising: directing a first beamof radiation along a first path onto a surface of the sample; directinga second beam of radiation along a second path onto a surface of thesample, said first and second beams producing a first and a secondilluminated spot on the sample surface, said first and secondilluminated spots separated by an offset; and receiving scatteredradiation from the first and second illuminated spots and focusing thescattered radiation to a detector.
 55. The method of claim 54, saidfirst or second beam having a point spread function with a spatialextent, wherein said offset is not less than said spatial extent but notgreater than three times the spatial extent.
 56. An optical method fordetecting anomalies of a sample, comprising: supplying a beam ofradiation at at least a first and a second wavelength; converting theradiation beam into a first beam at a first wavelength along a firstpath and a second beam at a second wavelength along a second path, saidtwo beams directed onto a surface of the sample; detecting radiation atthe first and second wavelengths by means of one or more detectors; andreceiving scattered radiation from the sample surface and originatingfrom the first and second beams and focusing the scattered radiation tosaid detectors.
 57. An optical method for detecting anomalies of asample, comprising: supplying a radiation beam; switching alternatelythe radiation beam between a first and a second path towards a surfaceof the sample; and receiving scattered radiation from the sample surfaceand originating from the beam along the first and second paths andfocusing the scattered radiation to a detector.
 58. The method of claim57, said method further comprising acquiring data from the detector at adata rate, wherein said switching is at a frequency of at least aboutthree times that of the data rate.
 59. An optical method for detectinganomalies of a sample, comprising: directing at least one beam ofradiation along a path onto a spot on a surface of the sample; receivingscattered radiation from the sample surface and originating from the atleast one beam and focusing the scattered radiation to a first detectorfor sensing anomalies; detecting a specular reflection of said at leastone beam in order to detect any change in height of the surface at thespot; and altering the path of the at least one beam in response to thedetected change in height of the surface at the spot to reduce positionerror of the spot caused by change in height of the surface at the spot.60. The method of claim 59, said directing including reflecting a beamof radiation from a mirror, wherein said altering includes rotating themirror.
 61. The system of claim 59, said specular reflection detectionproviding a position signal indicating height of the sample surface atthe spot, said altering including: providing a control signal having anamplitude equal to about half that of the position signal; and rotatingthe mirror by an amount proportional to the amplitude of the controlsignal.
 62. An optical method for detecting anomalies of a sample,comprising: directing at least one beam of radiation along a path onto aspot on a surface of the sample; collecting scattered radiation from thesample surface and originating from the at least one beam and conveyingthe scattered radiation to a first detector for sensing anomalies; andblocking scattered radiation towards the detector except for at leastone area having a wedge shape.
 63. The method of claim 62, said at leastone area having a shape of a butterfly with two wings, wherein scatteredradiation towards the detector is blocked at the two wings and passesbetween the wings.
 64. The method of claim 62, further comprisingaltering the size of the at least one area.
 65. The method of claim 64,said wedge shape having a wedge angle, said method employing an array ofwedge-shaped electrodes arranged around a center, each electrode in thearray overlapping at least one additional electrode, and a layer ofliquid crystal material between the array and said at least oneadditional electrode; said altering including: applying electricalpotentials across one or more electrodes in the array and the at leastone additional electrode to control radiation transmission throughsections of the liquid crystal layer to alter the wedge angle.
 66. Themethod of claim 65, said wedge-shaped electrodes having wedge angle(s)that are at least about 5 degrees.
 67. The method of claim 62, whereinsaid directing directs said at least one beam of radiation along a pathat an oblique angle to the sample surface.
 68. An optical system fordetecting anomalies of a sample, comprising: means for directing a beamof radiation along a path at an oblique angle onto a surface of thesample; a detector; and means including a curved mirrored surface forreceiving scattered radiation from the sample surface and originatingfrom the beam and for focusing the scattered radiation to said detector.69. The system of claim 68, wherein said mirrored surface has an axis ofsymmetry substantially coaxial with the path, defining an input aperturepositioned proximate to the sample surface to receive scatteredradiation therethrough from the sample surface.
 70. The system of claim69, said mirrored surface being a paraboloidal mirrored surface, themirrored surface reflecting radiation that passes through the inputaperture, said receiving and focusing means further including means forfocusing radiation reflected by the mirrored surface to the firstdetector.
 71. The system of claim 69, said mirrored surface being anellipsoidal mirrored surface, the mirrored surface reflecting andfocusing radiation that passes through the input aperture.
 72. Thesystem of claim 68, said path being at an angle within a range of about45 to 85 degrees to a normal direction to the sample surface.
 73. Thesystem of claim 68, further comprising a spatial filter between thedetector and the mirrored surface, said filter blocking scatteredradiation towards the detector except for at least one area having awedge shape.
 74. The system of claim 73, said filter having a shape of abutterfly with two wings, wherein scattered radiation towards thedetector is blocked by the two wings and passes between the wings. 75.The system of claim 73, wherein said filter is programmable to alter thesize, position or orientation of the at least one area.
 76. The systemof claim 75, said wedge shape having a wedge angle, said filtercomprising: an array of wedge-shaped electrodes arranged around acenter, each electrode in the array overlapping at least one additionalelectrode; a layer of liquid crystal material between the array and saidat least one additional electrode; and means for applying electricalpotentials across one or more electrodes in the array and the at leastone additional electrode to control radiation transmission throughsections of the liquid crystal layer to alter the wedge angle.
 77. Thesystem of claim 76, said wedge-shaped electrodes having wedge angle(s)that are at least about 5 degrees.
 78. An optical method for detectinganomalies of a sample, comprising: directing a beam of radiation along apath at an oblique angle onto a surface of the sample; providing acurved mirrored surface to receive scattered radiation from the samplesurface and originating from the beam; and detecting the scatteredradiation from the mirrored surface to detect anomalies of the sample.79. The method of claim 78, further including focusing radiationreflected by the mirrored surface to a detector.
 80. The method of claim78, said path being at an angle within a range of about 45 to 85 degreesto a normal direction to the sample surface.
 81. The method of claim 78,further comprising blocking scattered radiation towards the detectorexcept for at least one area having a wedge shape.
 82. The method ofclaim 81, wherein the blocking blocks scattered radiation towards thedetector over an area in the shape of a butterfly with two wings andpasses scattered radiation towards the detector between the wings. 83.The method of claim 81, further comprising changing the size of the atleast one area.
 84. The method of claim 83, said wedge shape having awedge angle, said blocking employing a filter comprising an array ofwedge-shaped electrodes arranged around a center, each electrode in thearray overlapping at least one additional electrode and a layer ofliquid crystal material between the array and said at least oneadditional electrode; said changing including: applying electricalpotentials across one or more electrodes in the array and the at leastone additional electrode to control radiation transmission throughsections of the liquid crystal layer to alter the wedge angle.